1. Field of the Invention
The present invention relates to a drain bolt and more particularly to a drain bolt with a magnet.
2. Description of the Prior Art
Gear boxes such as automotive transmission gear and differential gear have a tapped hole at the bottom to drain oil in the gear box. Normally the tapped hole is closed by a drain bolt screwed therein. In removing the oil in the gear box, the drain bolt is removed to allow the oil to drain through the tapped hole.
Some drain bolts have a magnet projecting into the gear box to attract iron particles produced as a result of wearing of gear. The drain bolt with a magnet can remove by attraction iron particles settled at the bottom of the gear box or contained in the oil.
Conventional drain bolts with a magnet are described as follows. In each of the drain bolts shown in FIGS. 10 through 12, a bolt body 1 has a circular recess 3 formed at the end of a male-threaded part 2 with a magnet 4 made of sintered ferrite embedded in the recess 3. In the case of FIG. 10, the magnet 4 is bonded to the bottom of the recess 3 with a bonding agent 5 which has good heat resistance and oil resistance. An annular caulking member 6 made of nonmagnetic material mounted on the inner circumference of the recess 3 has its end caulked against an end of the magnet 4. In the case of FIG. 11, the magnet 4 is bonded to the bottom of the recess 3 with the bonding agent 5. The end of the male-threaded part 2 is caulked at several locations against the end of tee magnet 4. In the case of FIG. 12, the magnet 4 is bonded to the bottom and the inner periphery of the recess 3 with the bonding agent 5. The drain bolt shown in FIG. 13 has the magnet 4 bonded to the end of the male-threaded part 2 with the bonding agent 5 and also reinforced by a set screw 7 which rigidly secures the magnet 4 to the end of the male-threaded part 2.
In these examples, the magnet 4 of sintered ferrite has low molding accuracy in terms of dimension and shape and this makes it difficult to obtain the desired shape and dimension that are optimum for assembly into the bolt body 1. The magnet 4 is also very fragile and easily broken by a strong external force. It is thus required that the magnet be fixed without exerting pressure on it as practically as possible. The common practice therefore is to interpose a jointing member such as the bonding agent 5, caulking member 6 and set screw 7 between the bolt body 1 and the magnet 4. The use of these jointing members results in an increase in the number of assembling processes.
Moreover, the magnet 4 of sintered ferrite is difficult to form into complicated shapes. For example, it cannot easily be formed into shapes in which the magnet 4 projects into the gear box to increase the iron particle attracting capability, in which the flux of the magnet 4 can effectively be utilized, or in which the surface area of the magnet 4 increases.